The metal organic frameworks (MOFs) are a highly crystalline complex compound constructed by specific materials. The MOFs usually form a coordination network in view of the microstructure. The coordination network is constituted primarily by “linkers” and “supportive clusters”, wherein the supportive clusters are connected by the linkers. Generally, the linkers are organic molecules forming “organic ligands”; and the supportive clusters are metal ions or metal clusters. The organic ligands and the supportive clusters are connected to form the second-building unit (SBU). Thus, the coordination of the organic ligands and the supportive clusters, such as the dimensional distribution and orientation of the organic ligands and the supportive clusters as well as the way of bonding, significantly affects the material features and the porous characteristics. Therefore, by modulating the coordination of the organic ligands and the supportive clusters, the metal organic frameworks with specific properties can be manufactured. The porous characteristics give the metal organic frameworks vast potential of application, such as gaseous storage, gas separation, sensing, adsorption, separation and other conventional industrial applications. Moreover, the potential of the metal organic frameworks in biomedical application such as drug delivery becomes conspicuous.
Biocatalysis system usually utilizes natural catalysts such as enzymes and living cells to be the catalysts. Nowadays, the biocatalysis system has been widely used in chemical industry, food industry and pharmaceutical industry. In biocatalysis system using enzymes for catalysis, in order to achieve better catalysis efficiency, but the cost is higher and the activity of enzyme is unstable. Thus, immobilizing enzymes on solid support is an effective solution. For example, the widely used protease, trypsin, is usually applied for digesting the proteins into the small fragment peptides for proteomic analysis or industrial processes. Conventionally, digesting proteins with trypsin solution usually requires a time-consuming procedure taking 18 hours or more, which definitely results in an inefficient procedure. Recently, people try to immobilize the protease on the solid support (carrier) to improve the performance of enzyme-to-substrate ratio (can be expressed as [E]/[S]), reusability, high hydrolytic catalytic ability, reduced reaction time; such that the overall bio-catalytic performance can be improved.
Therefore, the quality of immobilizing enzymes on the solid carrier is significantly relevant to the actual performance of the biocatalysis system. Specifically, a suitable solid carrier is selected according to the specific enzyme and ligand system chosen for the biocatalysis, and then the specific enzyme is immobilized on the solid carrier, such that the enzyme can play the role of catalyst to exert it bio-catalytic activity in the biocatalysis system. Moreover, such biocatalysis system also works well in non-physiological environments conditions, which extends the versatile usage of the biocatalysis system. For example, intensifying or increasing the density (multi-points) of the covalent bonding between the enzyme and solid carrier can increase the rigidity of the enzyme such that the enzyme can stably exert the catalytic ability even in a relatively harsh environment, such as extreme temperature conditions or non-aqueous solvent environment. Nevertheless, inappropriate enzyme immobilization may alter the conformation and the function of the enzyme, and it will detract the enzyme catalytic performance.
Variant materials are used as solid carriers, and the most widely used solid carriers include nanoparticles, polymers, and inorganic compounds. The solid carriers are used to construct the bioreactor. According to the recent research results by S. Hudson et al. (Chem. Rev., 2012, 112, 724-781), they points out that the repeatedly used mesoporous silicates show enzyme leaching effect, so as to the practical use of the mesoporous silicates for performing the bio-catalytic function in an bioreactor is invalid. In addition, some researchers utilize the post-synthetic modification approaches to modify the materials; however, concerning the stability between the solid carrier and the enzyme, these approaches are still controversial and waiting to be verified.
On the other hand, the current methods of immobilizing enzymes on the solid carriers usually adopt the approach of linking the enzyme with the solid carrier by covalent bonding, but seldom adopt the “adsorption approach” for enzyme immobilization. However, the aforementioned covalent bonding method generally requires complicated functional group modification to fully achieve the firmly immobilizing the enzyme to the solid carrier, the more advanced study can be found in research paper from M. E. Medina et al. (Adv. Mater., 2011, 23, 5283-5292). In brief, when the user adopts the physical adsorption method, the “pore size” of the porous material of the solid carriers becomes a critical factor for stabilizing the enzyme immobilization. In other words, the pore size is subjected to the hydrodynamic diameter of the target biomolecule (e.g., enzyme) when the physical adsorption method is adopted, and the pore size is typically larger than 3 nm to 8 nm. In general, the porous material can be categorized into three types in view of pore size (pore diameter): microporous material with the diameter smaller than 2 nm; macroporous material with the diameter larger than 50 nm; and mesoporous material with the diameter ranging from 2 nm to 50 nm, which is between that of the microporous material and the macroporous material.
Thus, for the current application of biomolecules immobilization, achieving a porous structure with mesoporous pores is a basic criterion of the porous materials, which can be identified as the “mesoporous material”. However, using the mesoporous material for the molecule adsorption or molecule immobilization usually needs complicated modification procedures (e.g., the chemical bonding or linking procedures) such that the mesoporous material can be more easily bonded with the biomolecule via chemical bonds, or adsorbed by the hydrophobicity attraction force, such that the biomolecules are immobilized or absorbed firmly on the mesoporous porous material. Thus, the absorption or immobilization can be improved for the specific biomolecule. The aforementioned “modification” procedures are unavoidably cumbersome and time-consuming, and additionally, the chemical processes of modification often consume large amounts of organic solvent and cause environmental problems.
On the other hand, whether the porous material can be widely applied for adsorbing or immobilizing specific molecules depends on the adsorption capability of the porous material for the molecules with different scales of sizes. At present, regarding the adsorption of larger size molecules (hereinafter referred to as macromolecules), such as the biomolecules of proteins, using the mesoporous material with oversized pore diameter results in the difficulty of retaining the macromolecules in the porous structure for further application procedures.
In response to the aforementioned technical demands of the porous materials, a few research results propose the concept and the technology of applying the metal-organic framework materials (MOFs) in biocatalysis system. The MOFs are used as the solid carrier or support for immobilizing the biomolecule in the biocatalysis system. For example, Y H. Shih et al. (ChemPlusChem., 2012, 11, 982-986), V. Lykourinou et al. (J. Am. Chem. Soc., 2011, 133, 10382-10385), Y. Chen et al. (J. Am. Chem. Soc., 2012, 134 (32), 13188-13191) respectively propose approaches to apply MOFs in biocatalysis system. Nevertheless, the aforementioned approaches or methods still relied on chemical bonding methods, which unavoidably have the aforementioned drawbacks of chemical bonding methods, such as complicated procedures, lengthy process, and environmental pollution threats.
Moreover, the presently available approaches for applying MOFs to immobilize macromolecules still have the disadvantages of significant morphological variations and small pores (usually less than 2 nm), which significantly limit the adsorption capability, separation ability or the macromolecule immobilization capability of MOFs. On the other hand, since the microporous material is so limited in the capability of adsorbing or immobilizing macromolecules, the knowledge of using the microporous material to adsorb the macromolecule is rare. Only few researches partially disclose the applications. For example, V. Lykourinou and Y. Chen, et al, propose methods of using microporous MOFs to physically adsorb macromolecule in the microporous structures of MOFs. However, although the microporous MOFs materials are used, regarding the size of the target molecule (macromolecules), the target molecule cannot fully enter the porous structure of microporous MOFs and thus cannot be retained therein. Thus, the adsorption is limited to a “surface adsorption”. In addition, the adsorption of the aforementioned techniques is a lengthy process, which consumes about to 40 hours to 60 hours to complete the adsorption step and it is very time-consuming.